Cyclotron ion source



March 8, 1966 R JEPPSQN 3,239,707

CYCLOTRON ION SOURCE Filed March 1, 1962 2 Sheets-Sheet 1 2 Sheets-Sheet 2 M. R. JEPPSON CYCLOTRON 1cm SOURCE ZIIIIIIIIIIIIII'IIIII March 8, 1966 Filed March 1, 1962 N mi United States Patent 3,239,7fi7 CYCLOTRGN ION SOURCE Morris R. Jeppson, Alamo, Calif., assiguor, by mesne assignments, to High Voltage Engineering Corporation, Burlington, Mass, a corporation of Massachusetts Filed Mar. 1, E62, Ser. No. 176,725 12 Claims. (Cl. 313-62) This invention relates to cyclotrons and more particularly to a new and improved ion source therefor.

- Basically, a cyclotron is a magnetic resonance accelerator wherein electrified particles (such as pro-tons or helium nuclei) are released in the region between two large, flat, hollow, semi-circular segments of thin metal placed with their diametric edges closely parallel. These segments, called dees, have impressed therebetween a high frequency potential. The resulting rapidly alternating field causes a free charged particle residing in the inter-dee space to be pulled first one way and then the other. A strong uniform magnetic field, perpendicular to the plane of the dees, causes a charged particle darting into a dee to follow a semi-circular path of radius proportional to its speed, and to re-enter the inter-dee space on the other side of the center. The cyclic time of flight of any given charged particle is a function of the intensity of the magnetic field. The magnetic field, then, is adjusted so that this time is equal to one-half the electric oscillating period, allowing the particle to emerge into the electric field so directed as to pull it in the direction it is already going, thereby increasing its speed at every crossing. Thus, starting near the center, the particle spirals outward increasing speed at each half turn and finally escaping into a receptacle near the outer edge.

For various reasons, provision of a copious supply of charged particles at the center of the cyclotron has proved to be a troublesome and a never satisfactorily achieved objective. The limited space available presents a serious design restriction. The production of ions by the discharge of a hot tungsten filament in the presence of relatively high pressure gas is incompatible with vacuum requirements. And the presence of high electric and magnetic fields obviates any possibility of etricient ion injection in the proper plane and direction.

Most commonly, cyclotron ion sources, as represented by the current state of the art, are of the so-called hooded arc variety. Such an arrangement provides a gas supply conduit which extends to the center of the cyclotron and hasa tungsten filament disposed therein. Protruding upwardly therefrom into the center of the dees is a hollow cylinder into which a small aperture has been cut. Ions 7 produced by the tungsten filament travel up the hollow cylinder and are drawn therefrom through the aperture by the electric field of the dees. To increase the field strength between dee and ion discharge cylinder, the upper and lower edges of the dee are often extended by metal lips or feelers. This has the advantage of permitting the ion source to be moved about until the position is such as to maximize the number of particles reaching the deflecting system. The unwanted vertical component given to the ions velocity by this scheme is considerably reduced by replacing the lips by two vertical bars or pullers across the opening of the dee so as to form an elementary grid that helps extract and launch the ions. However, while the procedure does not produce as large a vertical velocity component, it leaves less freedom in the position of the ion source. Further complicating ion source design is the presence of strong electric and magnetic fields which make it desirable to remotely locate source equipment and auxiliaries. Also because of the intense radioactivity produced by very high current cyclotrons when accelerating deuterons, maintenance and access Patented Mar. 8, 1966 become serious problems. The source equipment, ideally, would be located not only outside of the electric and magnetic fields, but also outside of the shielding. Still another consideration in ion source design is the application thereof to the recently developed spiral ridge cyclotron. The excellent versatility of this device, which provides variable beam energy and comprehends acceptance of all ions, has made it a standard of university research. The troublesome electric and magnetic fields of the conventional cyclotron, in the spiral ridge version, present further problems in that they become variable fields thereby creating even greater demands upon the ion source.

There is, then, a current need to improve and provide greater versatility to cyclotron ion sources. From the foregoing discussion it is seen that, of necessity, any conventional approach toward such an improvement must be predicated upon a compromise of various incompatible criteria. My invention entertains a new concept which represents a radical departure from conventional design; that is, the concept of projecting a neutral beam through the troublesome electric and magnetic fields and into a charge exchange device located within the cyclotron. A neutral beam, as referred to herein is defined as a beam of gas molecules (such as hydrogen or helium) that is generated by ionizing a quantity of such gas, accelerating and collimating the ions thus formed, and passing the ion beam through a charge exchange chamber to elfect neutralization thereof. The gas molecules, in such a neutral beam retain the energy of the accelerated ions from which they originated and continue to drift in space as a collimated beam. The neutral beam, thus defined, is not to be confused with an ordinary gas jet or stream which may be formed by gas under pressure escaping through an aperture.

It is a principal object of my invention to provide a. new and improved cyclotron ion source.

It is another object of my invention to provide a cyclotron ion source that requires a minimum of space within the cyclotron vacuum chamber.

It is another object of my invention to provide a cyclotron ion source that is independent of and is unaffected by the cyclotron electric and magnetic fields.

It'is another object of my invention to provide a cyclotron ion source that permits the injection of ions therein in the optimum direction to achieve maximum trapping etliciency.

It is another object of my invention to provide a cyclotron ion source that emits a more copious supply of ions than conventional ion sources while maintaining a minimum of vacuum contamination effects.

It is another object of my invention to provide a cyclotron ion source wherein said source equipment is remotely located therefrom, thus permitting safe access and maintenance where intense radioactivity is produced.

It is another object of my invention to provide a unique cyclotron ion source, such ion source utilizing a neutral beam and a charge exchange device and being particularly adapted to meet the rigorous demands of the so-called spiral ridge cyclotron.

Other objects and features of my invention as well as a clearer understanding thereof will be obtained from an examination of the following detailed description. While such description relates to one particular illustrative embodiment, it is to be understood that my invention is not limited thereto, said embodiment being by way of illustration only, the full scope and nature of the inventive concepts contained therein being defined in the appended claims.

In the accompanying drawings, wherein like elements are given like reference numerals throughout:

FIGURE 1 illustrates an isometric, partially cut away, view of a cyclotron in combination with an ion source of the type comprehended by my invention:

FIGURE 2 illustrates a sectional view of such a cyclotron and ion source; and

FIGURES 3, 4 and 5 illustrate various charge exchange devices used in accordance with my invention.

Reference is now made to the partially cut away isometric view of cyclotron 8 illustrated in FIGURE 1. A neutral beam 16 is generated in neutral beam source 7 and directed via conduit 12 to cyclotron vacuum chamber 10. The diameter of the neutral beam may be relatively small, that is in the order of one-quarter or one-eighth of an inch. The coupling aperture between conduit 12 and vacuum chamber 10, then, being commensurately small, causes a minimium disturbance in the cyclotron vacuum system. A strong transverse magnetic field is supplied by magnets 11 (the top magnet is omitted in FIGURE 1 for clarity of illustration), and a periodic electric field is supplied across dees 13. Neutral beam 16 is projected to the center of the cyclotron, and, having no electric charge is unaffected by the strong electric and magnetic fields residing therein. Neutral beam 16 is so aligned as to pass through aperture 18 of charge exchange chamber 14. Charge exchange chamber 14. Charge exchange chamber 14 contains a discrete charge exchange gas supplied thereto by conduit 15. Ions created therein emerge as beam 17 and, being electrically charged are affected by the transverse magnetic field and begin to travel in a cyclic pattern as shown. The nature of ion beam 17, that is such characteristics as density, energy, direction, and the like, may be controlled externally in the neutral beam source and by properly orienting the neutral beam and charge exchange chamber. Although, in the illustration of FIGURE 1, neutral beam 16 is projected between dees 13, it would be equally feasible to inject it through an aperture in one of said dees, should effective trapping geometry so dictate. It is readily seen, therefore, from an inspection of the arrangement of FIGURE 1 that there is no restriction on ion source location; that a copious supply of ions are ejected from the cyclotron exchange chamber in a horizontal plane and in a direction best adapted to effect maximum trapping efficiency; and that the source from which the ions are supplied and controlled may be located as remotely as required.

A sectional view illustrating the above described apparatus in greater detail is presented in FIGURE 2. In particular, reference is made to neutral beam source 7 which is described as follows. Vacuum chamber 21 encloses the various electrodes and is continually pumped to insure the maintenance of an adequate vacuum throughout the system. Ionizing gas is supplied to ionization chamber 23 through gas supply conduit 22. This may be any gas appropriate for ionization such as helium or hydrogen. Discharge of filament 24 in the presence of such gas results in the ionization thereof, said ions being concentrated near aperture plate 26 by the field of electromagnet 2 5. Extraction electrode 27 is maintained at a potential adapted to extract ions from ionization chamber 23. The ions thus extracted are collimated by focussing electrode 28, accelerated to the desired velocity by accelerating electrode 29, and directed through charge exchange canal 30. A controlled amount of gas such as hydrogen or carbon dioxide is maintained in said charge exchange canal through charge exchange gas line 31.

In charge exchange canal 30 the ion beam passing therethrough encounters an amount of hydrogen, carbon dioxide, or other applicable gas, which, though relatively small, is sufficient to cause the attachment of a single electron to most of the ions, thus producing a beam of neutral particles which proceeds to cyclotron 8 via vacuum chamber aperture 32 and conduit 12. Magnets 35, 36 are provided to deflect any charged particles emanating from exchange canal 30 away from vacuum chamber aperture 32. The neutral beam is aligned to project through aperture 20 of charge exchange chamber 19 located within cyclotron 8. The charge exchange between said neutral beam and the gas residing in charge exchange chamber 19 produces the desired supply of ions, said ions emanating therefrom with substantially the same energy as the incoming neutral beam and in an optimum direction to provide efiicient trapping as hereinbefore described with reference to FIGURE 1. Conduit 12 may be any practical length, since the neutral beam, having no space charge effects, will retain its characteristics and collimation until it is intercepted by some dispersing object. It is, of course, necessary that the conduit be straight, since the beam, in its neutral state can not be bent or deflected.

Referring now to FIGURES 3, 4 and 5, there are illustrated various charge exchange devices adapted for use within a cyclotron in accordance with the principles of my invention. FIGURE 3 illustrate vertically disposed cylindrical container 19 affixed to the bottom of vacuum chamber 10. Charge exchange gas such as hydrogen, carbon dioxide or oxygen is supplied thereto by conduit 15. Apertures 20 allow the passage therethrough neutral beam 16, said neutral beam being ionized therein by interaction with the charge exchange gas and emerging as ion beam 17. An alternate arrangement of such a gas charge exchange device is illustrated by FIGURE 4. In this embodiment an exchange canal 14 is provided having an aperture 18 in each end thereof. The geometry of this particular arrangement provides a longer interaction path for the neutral beam thus enhancing the probability of complete ionization thereof. Another embodiment that can be used to advantage where high power ion beams are not required and where positive ion beams are being accelerated is illustrated by FIGURE 5. In this embodiment, a frame 32 holds metal foil 33 in intercepting relationship with neutral beam 16. Metal foil 33 may be aluminum or any other electron stripping material that will ionize a neutral beam passing therethrough. This arrangement has the advantage of eliminating the charge exchange gas supply, but of course must be restricted to low power devices.

There has thus been described novel apparatus for providing a cyclotron ion source whereby the several objects of my invention are accomplished. While such apparatus has been described with reference to specific structure, the inventive concepts residing therein are not to be taken as being limited thereto. Since various modifications and applications of my invention will occur to those skilled in the art, it is intended that the spirit and scope thereof be defined only by appended claims.

I claim:

1. In combination with a cyclotron, an ion source comprising neutral beam generating means, said neutral beam generating means being disposed externally of said cyclotron, neutral beam ionizing means, said neutral beam ionizing means being disposed within said cyclotron, and means for directing said neutral beam through said neutral beam ionizing means.

2. In a cyclotron, an ion source comprising (1) a remotely located means for generating a neutral beam including, in a vacuum chamber,

(a) a supply of gas to be ionized, (b) means for ionizing said gas, (c) a first electrode adapted to focus and collimate said ions, (d) a second electrode adapted to accelerate said 5 (e) a charge exchange canal aligned to accept the passage therethrough of the ion beam formed thereby, and (f) a quantity of charge exchange gas disposed within said charge exchange canal,

(2) a charge exchange chamber, said chamber being centrally located within said cyclotron and consisting of a gas filled hollow cylindrical member having apertures therein, said apertures being positioned to admit the projection of said neutral beam therethrough, and

(3) conduit means for coupling said means for generating a neutral beam with said cyclotron, said conduit means being adapted to align said neutral beam with said charge exchange chamber apertures.

3. An ion source as defined in claim 1 wherein said neutral beam generating means comprising in a vacuum chamber, a supply of gas to be ionized, means adapted to ionize said gas, a first electrode adapted to focus and collimate into a beam the ions thus formed, a second electrode adapted to accelerate said ion beam, and a charge exchange canal, said charge exchange canal having a quantity of gas disposed therein and being positioned such that said accelerated ion beam is directed therethrough.

4. An ion source as defined in claim 3 wherein said gas to be ionized consists of helium.

5. An ion source as defined in claim 3 wherein said gas to be ionized consists of hydrogen.

6. An ion source as defined in claim 3 wherein said gas disposed in said charge exchange canal comprises carbon dioxide.

7. An ion source as defined in claim 3 wherein said gas disposed in said charge exchange canal comprises hydrogen.

8. An ion source as defined in claim 1 wherein said neutral beam ionizing means comprises a vertically disposed hollow cylindrical member having apertures therein, said apertures being aligned to permit the passage of said neutral beam therethrough, said cylindrical member having a quantity of gas residing therein.

9. An ion source as defined in claim 8 wherein said quantity of gas residing in said cylindrical member consists of carbon dioxide.

19. An ion source as defined in claim 1 wherein said neutral beam ionizing means comprises a thin metal foil member, said foil member being in intercepting relationship With said neutral beam and adapted to strip electrons therefrom.

11. An ion source as defined in claim 10 wherein said metal foil member is fabricated of aluminum.

12. An ion source as defined in claim 1 wherein said means for directing said neutral beam through said neutral beam ionizing means comprises a straight conduit member, said conduit member being arranged to couple said neutral beam generator near to said cyclotron.

References Cited by the Examiner UNITED STATES PATENTS 1,948,384 2/1934 Lawrence 313-63 X 2,219,033 10/1940 Kuhm et a1. 31363 X 2,789,221 4/1957 Tobias 313-62 HERMAN KARL SAALBACH, Primary Examiner. GEORGE N. WESTBY, ARTHUR GAUSS, Examiners. 

1. IN COMBINATION WITH A CYCLOTRON, AN ION SOURCE COMPRISING NEUTRAL BEAM GENERATING MEANS, SAID NEUTRAL BEAM GENERATING MEANS BEING DISPOSED EXTERNALLY OF SAID CYCLOTRON NEUTRAL BEAM IONIZING MEANS, SAID NEUTRAL BEAM IONIZING MEANS BEING DISPOSED WITHIN SAID CYCLOTRON, AND MEANS FOR DIRECTING SAID NEUTRAL BEAM THROUGH SAID NEUTRAL BEAM IONIZING MEANS. 